Axial piston machine having a swashplate design

ABSTRACT

An axial piston machine having a swashplate design includes a cylinder drum that is rotatably supported in a housing and that has a plurality of cylinder bores, which have pistons that can be slid longitudinally in said cylinder bores and that are supported on a swashplate. The aim of the disclosure is to achieve an improvement in the efficiency with a short length design by means of reducing the lost forces. The aim is achieved in that no force components act on the pistons in the radial direction.

The invention starts from an axial piston machine having a swashplate design, which has the features from the preamble of claim 1.

Known axial piston machines of this type have a cylinder drum, which is rotatably supported in a housing and has a plurality of cylinder bores containing pistons, which can be moved longitudinally in said cylinder bores and are supported on a swashplate.

EP 0 767 864 B1 describes an embodiment of an axial piston machine having a swashplate design, said embodiment being preferred in practice. During the rotary motion of the cylinder drum, the pistons perform a translatory stroke motion in the cylinders. During this process, the piston heads are in continuous contact with the swashplate, which is arranged at a certain angle to the cylinder drum axis. Each piston performs one stroke for each revolution of the cylinder drum. The torque results from the embodiment of the piston head/swashplate pair at the cylinder drum. The piston head is usually designed as a point, a spherical surface or a sliding shoe, and the surface of the swashplate is flat. Neglecting the friction forces, the reaction force of the swashplate is a perpendicular force, i.e. a force acting parallel to the swashplate axis, which takes effect at the center of the piston head and acts on the cylinder walls in the cylinder drum via the piston. The swashplate reaction force taking effect in the piston head can be resolved into a force component in a circumferential direction and a force component in a radial direction. The force component in a circumferential direction produces the torque. In the case of a flat swivel cradle, a force component in a radial direction is always produced as well. This accordingly increases the transverse loads in the piston and in the cylinder and leads to a proportionate reduction in the torque that can be transmitted. As a result, the piston is subjected to bending stress. When each piston performs a suction and a compression stroke during one complete revolution of the cylinder drum through 360°, the pistons are in each case in one of the dead center positions thereof as the piston chambers switch over between the suction and the pressure side. During this phase, the radial force components are at a maximum, whereas they are zero after a further quarter rotation. Thus, the radial force components, which exert a negative effect on the pistons, occur especially as they pass through the dead center positions, with the result that the sum of the radial force components, as a lost force, both impose high stress on the piston/cylinder drum pair and have a negative effect on the torque and thus detract from the efficiency of the axial piston machine. Owing to the above-described manner of support for the pistons on the swashplate by means of a ball joint and sliding shoes, the length of the axial piston machine is relatively great.

It is the underlying object of the invention disclosure to develop an axial piston machine having a swashplate design and having the features from the preamble of claim 1 in such a way that, together with a short length, an improvement in efficiency is achieved through a reduction in the lost forces.

This object is achieved in the case of an axial piston machine having a swashplate design and having the features from the preamble through additional provision of the features from the characterizing part of claim 1.

In an axial piston machine according to the invention disclosure having a swashplate design, no force component in a radial direction acts on the pistons.

By virtue of the fact that the force component in a radial direction is eliminated, the forces acting on the piston and the piston guide in the cylinder are reduced by this component. The load on the components is thus reduced, and the torque is not counteracted by any transverse forces. As a result, there is an increase in the efficiency of the axial piston machine.

Advantageous embodiments of an axial piston machine according to the invention disclosure having a swashplate design are indicated in the dependent claims.

According to a particularly preferred embodiment of the present invention disclosure, the swashplate has a running surface which is embodied in such a way that the longitudinal axis of the piston is perpendicular to the running surface in a radial direction at the respective point of contact. As a result, the piston is subject only to a force component in the circumferential or running direction, and a radial force component does not occur here. The swashplate surface has curved areas, which make it a simple matter in terms of production engineering to form the running surface of the piston perpendicularly to the longitudinal axis thereof.

The running surface is preferably designed to be without a curvature in a radial direction, ensuring that the piston slides over the running surface without guidance and with point contact at the respective point of contact.

If the running surface has a circumferential groove, the piston can be guided in a linear manner. This embodiment proves to be particularly advantageous if the piston head is designed as a ball and the ball rolls directly on the running surface of the swashplate.

It is advantageous to make the swashplate end of the piston rotationally symmetrical. In this case, the piston head does not rest flat on the swashplate, and there is therefore no need for hydrostatic load relief between the piston head and the swashplate.

A particularly advantageous embodiment is the spherical design of the swashplate end of the piston. In this case, the overall length of the axial piston machine is less.

If the swashplate end of the piston is designed as a ball, the piston and hence the axial piston machine are particularly short, the production of the piston is particularly simple and economical, the length of extension of the piston out of the cylinder is reduced, and disadvantages associated with a sliding shoe solution, such as the tendency of the sliding shoes to tilt, are eliminated. A high-precision smooth design of the sliding plate surface is rendered unnecessary by the rotationally symmetrical or spherical design of the piston head.

Embodiments of an axial piston machine according to the invention having a swashplate design are illustrated in the drawing. The invention will now be explained in greater detail with reference to the figures of these drawings, of which

FIG. 1 shows a longitudinal section through an axial piston machine according to the invention having a swashplate design,

FIG. 2 shows a longitudinal section through a piston/sliding shoe joint on a swashplate with a flat surface in accordance with the prior art, and

FIG. 3 shows a longitudinal section through a piston having a spherical head on a swashplate according to the invention disclosure.

The axial piston machine 1 having a swashplate design, which is illustrated in FIG. 1, has a drive mechanism 2, which is arranged in a housing 3. As essential components, the drive mechanism 2 comprises a rotatably supported drive shaft 4 with a cylinder drum 5 connected for conjoint rotation therewith. The cylinder drum 5 has axially extending cylinder bores 6 arranged on a pitch circle, in which pistons 7 are arranged in a manner which allows longitudinal movement. Together with the cylinder drum 5, the pistons 7 each delimit a displacement chamber 8, which can be connected via a control disk 9 to a pressure or suction port (not shown). The pistons 7, which are guided with the ability for longitudinal movement in the cylinder bores 6, are preferably of cylindrical design. The ends of the pistons 7 which are remote from the cylinder drum are each supported via a joint 10 on a swashplate 11. The swashplate 11 is penetrated by the drive shaft 4. This figure does not show that it is designed as a swivel cradle of semi-cylindrical cross section and is arranged in a manner which allows it to be swiveled in a spherical bearing and fixed in the respective swivel position by means of an adjusting device 12. A running surface 13 is formed as a flat surface on the side of said swash plate which faces the cylinder drum 5.

When the drive shaft 4 is rotated, the cylinder drum 5 together with the pistons 7 also rotates, owing to the connection for conjoint rotation. If the swashplate 11 has been swiveled into an oblique position relative to the cylinder drum 5 by actuation of the adjusting device 12, the pistons 7 perform stroke motions. For each complete revolution of the cylinder drum 5, each piston 7 performs a suction and a compression stroke, with corresponding oil flows being produced, these being supplied and discharged via orifice ducts (not shown), the control disk 9 and a pressure and suction duct (not shown).

FIGS. 2 and 3 show joints of different designs, via which the ends of the pistons which are remote from the cylinder drum are supported on nonadjustable swashplates of different designs.

FIG. 2 shows a joint 20 in the form of a piston/sliding shoe pair supported on a flat surface 21 of a nonadjustable swashplate 22. The joint 20 comprises a piston 24 and a sliding shoe 30, which is supported over an extended area. The piston 24 has a piston shank 25, a piston neck 26 and a spherical piston head 27. The piston head 27 is rotatably supported in a partially spherical recess 29 in the sliding shoe 30.

The spherical recess 29 fits around an upper half of the piston head 27. For hydrostatic load relief of the piston/sliding shoe joint 20, the piston 24 has a through duct 31 along the central axis, said duct leading into the partially spherical recess 29 in the sliding shoe 30. The sliding shoe 29 has a sliding shoe base 31. Arranged in the sliding shoe base 32 is a pressure pocket 34 for hydrostatic load relief of the sliding shoe 30, said pocket being connected by a through duct 33 to the partially spherical recess 29.

During operation, the sliding shoes 20 run directly on the flat surface 21 of the swashplate 22. During one complete revolution of the cylinder drum through 360°, each piston 24 performs a suction and a compression stroke. As the piston chambers switch over between the suction and the pressure side, the pistons 24 are in each case in one of the dead center positions thereof, TDC at 180° or BDC at 0° or 360°. The reaction force F_(S) of the swashplate 22, which takes effect in the piston head 27, has a force component in a circumferential direction F_(U), which is responsible for producing the torque. However, a force component in the normal direction F_(N) and a force component in a radial direction F_(R), which imposes a load on the piston in the transverse direction, are also produced. The radial force component causes a proportionate reduction in the conversion of the torque into the force F_(S). In the region of top dead center and of bottom dead center, the radial force component F_(R) is at a maximum while, after a further quarter rotation, at 90° or 270°, the radial force component F_(R) is zero. The force component F_(R) has a disturbing effect because the center of the ball joint is always outside the cylinder bore and, as a result, the piston 24 is subjected to bending stress.

There are various piston/sliding shoe variants, whether the variant described above or, for example, the variant in which the sliding shoe has a spherical head on the piston side and the piston has a spherical recess on the sliding-shoe side. An appropriate piston/sliding shoe pair is chosen, depending on the application, and in all cases ensures sliding support on the surface of the swashplate. In addition to design requirements relating to the material properties of the piston and the sliding shoe, the piston/sliding shoe pair also has to meet demands for the lowest possible wear and the capacity to bear high loads. The radial force component F_(R) has a negative effect in this respect.

FIG. 3 shows a joint 40, which is supported on a curved surface 41, conforming to the invention, of a nonadjustable swashplate 42, said joint comprising a piston 44 and a ball 48, which rolls on the swashplate 42. The piston 44 has a piston shank 45. The swashplate end 46 of the piston 44 has a partially spherical recess 47, in which the ball 48 is rotatably supported.

The partially spherical recess 47 fits around a lower half of the ball 48 and ends with a tubular edge 49. For hydrostatic load relief, the piston 44 has a through duct 51 along the central axis, said duct leading into the partially spherical recess 47 in the piston end 46. The swashplate 42 has a running surface 53 which does not slope in a radial direction in the running direction of the pistons 44 and therefore extends in a radial direction perpendicularly to the longitudinal axis of the piston 44 at the respective point of contact of said piston. In this arrangement, the running surface 53 can be designed without a curvature in a radial direction or can have a circumferential groove 54.

By virtue of the running surface geometry, the reaction force F_(S) of the swashplate 42, which takes effect at the center of the ball 48 during operation, can be resolved only into a normal force F_(N) and a circumferential force F_(U) which is responsible for producing the torque. By virtue of the running surface geometry, radial forces do not occur at any time.

A radial force component, which increases the friction losses and wear phenomena at the piston/cylinder pair in the case of a piston/sliding shoe joint, does not occur here.

The running surface 53 can be free from curvature in a radial direction or can be embodied with a circumferential groove 54. With the groove 54 in the running surface 53, the piston is additionally provided with linear guidance. The groove can be of spherical or v-shaped design.

It could be advantageous from the point of view of cost and functionality to incorporate an independent running surface, in which case it would also be possible to embody the running surface 53 as an independent component. The stroke of the piston 44 can be defined in any desired manner by the stroke curve as a function of the angle of rotation. 

1. An axial piston machine comprising: a housing, a swashplate, and a cylinder drum rotatably supported in the housing, the cylinder drum defining a plurality of cylinder bores containing pistons, which are configured to move longitudinally in the cylinder bores and are supported on the swashplate, wherein no force component in a radial direction acts on the pistons.
 2. The axial piston machine as claimed in claim 1, wherein the swashplate has a running surface that is perpendicular to a longitudinal axis of the piston at the respective point of contact.
 3. The axial piston machine as claimed in claim 2, wherein the running surface is configured without a curvature.
 4. The axial piston machine as claimed in claim 2, wherein the running surface has a circumferential groove, in which the piston is guided in a linear manner.
 5. The axial piston machine as claimed in claim 1, wherein the piston has a swashplate end that is of rotationally symmetrical design.
 6. The axial piston machine as claimed in claim 5, wherein the swashplate end of the piston is of spherical design.
 7. The axial piston machine as claimed in claim 5, wherein the swashplate end includes a ball that is rotatably supported in the swashplate end of the piston. 